(1) Field of the Invention
The present invention relates to a measurement device for determining the angular positions about at least one pivot axis of a rotorcraft blade element relative to a hub of a main lift and propulsion rotor, or of an anti-torque rotor.
(2) Description of Related Art
During the rotation of the hub of a rotor of a rotorcraft, e.g. such as a helicopter, a blade element is generally suitable for pivoting about three pivot axes in a rotating orthogonal reference frame associated with the hub.
A first pivot axis is referred to as the “pitch” axis and enables the aerodynamic angle of incidence of the blade to be modified, and consequently makes it possible to modify the aerodynamic forces on the blade that give rise to the lift and the traction that the rotor exerts on the rotorcraft. Such a pitch axis thus extends substantially parallel to a longitudinal direction corresponding to the span of the blade element.
A second pivot axis is referred to as the “flapping” axis and it enables a free end of the blade element to move substantially perpendicularly relative to the plane of rotation of the rotor. Such a flapping axis is thus substantially contained in the plane of rotation of the blades of the rotor.
A third pivot axis is referred to as the “lead/lag” axis and is arranged substantially perpendicularly relative to the first and second pivot axes of the blade element.
The invention thus seeks more particularly to provide a measurement device making it possible to sense and store automatically data about these angular positions that vary throughout the rotation of the rotor, including during each revolution.
It is of particular interest to be able to evaluate the pivoting movements of a rotor blade element during a predetermined limited period in time or throughout the lifetime of the rotorcraft. Such measurement of these pivoting movements, during normal flight conditions, makes it possible to understand and evaluate very accurately the dynamic stresses to which the rotor of a rotorcraft is subjected.
In general manner, in order to measure the angular positions of a blade element, it is known to use devices for measuring the positions of a rotorcraft blade element that carries targets or patterns of randomly distributed points. Such targets or patterns are then fitted on the blade element in order to identify the flapping angle of blade elements during each rotation of a rotor. Such measurement devices are described in particular in Documents U.S. Pat. No. 4,604,526 and GB 1 116 748, or indeed in a technical publication entitled “Application of image pattern correlation for non-intrusive deformation measurements of fast rotating objects on aircraft”, given the reference XP040515416 in the database of non-patent works in the European Patent Office, and written by Fritz Boden, Kai Bodensiek, and Boleslaw Stasicki.
Although such a technical publication discloses the use of a camera for taking photos of a pattern placed on a blade element, that pattern of randomly arranged points does not enable the various angular positions of the blade element under consideration to be measured quickly and simply.
Furthermore, the camera is arranged on a stationary portion of the rotorcraft fuselage and not in a rotating reference frame such as the hub of a rotor. In addition, such a measurement device also requires a stroboscopic lamp in order to facilitate the taking of the photos of the pattern, and their subsequent analysis.
Thus, such a measurement device is very complex to implement for the purpose of identifying the angular positions of the blade element, and it does not enable these angular positions to be measured over a complete revolution of the rotor.
Furthermore, use has also been made of optical sensors such as described in Documents JP 2010/149602 or US 2014/226153. Those documents propose measurement devices in which a light source is placed on the blade and a detector is placed on the hub. Such a solution can thus serve only for measuring the flapping angle of the blade element.
Such a device also requires the blade element to be fitted with a source of light that needs to be housed in and secured to the blade element. Incorporating such a light source thus makes it necessary to modify the structural design of the blade element.
Furthermore, it is also necessary to convey electricity to the light source so as to enable it to emit light. It is particularly difficult to take electricity to a rotorcraft rotor, i.e. in a rotating reference frame. Furthermore, relative movements between the various parts of the rotor can lead to wear in electrical contacts and render such a solution unreliable over time.
In addition, however small such a light source may be, it gives rise to additional rotating weight and thus requires the rotor to be rebalanced.